Powder compacting method, powder compacting apparatus and method for producing rare earth magnet

ABSTRACT

A powder compacting method includes the steps of: providing a powder material; loading the powder material into a cavity; uniaxially pressing the powder material, which has been loaded into the cavity, between two opposed press surfaces, thereby obtaining a compact, wherein at least one of the two press surfaces is deformed elastically under a compacting pressure when contacting with the powder material in the cavity; and unloading the compact from the cavity. According to this powder compacting method, even when the powder material has a non-uniform fill density distribution, a compact with a uniform density distribution can be obtained at a high productivity.

TECHNICAL FIELD

The present invention relates to a method and apparatus for compacting apowder and a method for producing a magnet. More particularly, thepresent invention relates to a powder compacting method and apparatusthat can be used effectively to compact a rare earth alloy powder andalso relates to a method for producing a magnet out of a rare earthalloy powder.

BACKGROUND ART

A powder compacting technique is used to manufacture various types ofparts to be made of a ceramic or a metal. For example, a sintered bodyof a ceramic or a metal is manufactured by sintering a compact that hasbeen obtained in a predetermined shape by subjecting a powder materialto a powder compaction process. By subjecting the sintered bodies to afinishing process thereafter to adjust the sizes and shapes thereof,final products are obtained.

Generally speaking, the quality of a sintered body (in terms of physicalproperties or configuration) is determined by the quality of a compact.Also, the compactability depends on the particle size distribution orthe particle shapes of the powder material. For these reasons, variouspowder compacting methods have been proposed so far to obtain a compactof quality.

For example, a sintered magnet of a rare earth alloy may be produced byperforming the process steps of:

-   -   (1) melting a material metal at a high temperature to obtain an        ingot of a rare earth alloy with a predetermined composition;    -   (2) pulverizing this alloy ingot to obtain a rare earth alloy        powder of a small size;    -   (3) compacting the resultant alloy powder (to the surface of        which a lubricant is added depending on the necessity) under a        magnetic field to obtain a compact in a predetermined shape;    -   (4) sintering this compact at a high temperature (e.g., about        1,000° C. or more) to obtain a sintered magnet;    -   (5) further subjecting the resultant sintered magnet to a heat        treatment called an “aging treatment” to improve the magnetic        properties thereof; and    -   (6) grinding the surface of the sintered magnet to adjust the        sizes and shapes thereof.

In compacting an alloy (or magnet) powder material for use to producethe magnet described above, the orientation directions of the alloyparticles need to be aligned with a predetermined direction under amagnetic field. It is known that a rare earth sintered magnet withexcellent magnetic properties can be obtained by using an alloy powderthat has been prepared by a strip casting process. However, it isparticularly difficult to obtain a compact of quality from this alloypowder. This is because the alloy powder obtained by a strip castingprocess or any other rapid cooling process has a small mean particlesize (e.g., about 2 μm to about 5 μm), a narrow particle sizedistribution and a low flowability (or compactability). It should benoted that the “mean particle size” means herein a mass median diameterunless stated otherwise.

To produce a rare earth sintered magnet with excellent magneticproperties, the present inventors tested various conventional powdercompacting methods to discover that those methods had the followingproblem. This problem will be described with reference to FIGS. 13( b)and 13(c). It should be noted that the feature of an inventive powdercompacting method as shown in FIG. 13( a) will be described later.

Suppose an alloy powder material, prepared by a strip casting process,is loaded into a cavity and pressed by an upper punch and a lower punch(typically made of a metal (e.g., SUS 304)) in accordance with a normaluniaxial compacting method (typically a die-pressing method). In thatcase, if the alloy powder material 10 has a fill density (or loadingweight) distribution as shown in FIG. 13( b) (where H indicates a highdensity and L indicates a low density), then the resultant compact 20should also have a non-uniform density distribution corresponding to thefill density distribution. Also, even if the cavity is filled with thealloy powder material at a sufficiently uniform density, the alloypowder material may still show some variation in its fill density whensubjected to a magnetic field alignment process during the pressingprocess. Such a variation is caused by the field strength (or fluxdensity) distribution during the magnetic field alignment process. Ahigher pressure is normally applied to a portion with a higher filldensity. Accordingly, when the alloy powder material is subjected to thepressing process, such a variation in its density is amplified. And ifsuch a density variation is significant, then the compact may crack,chip or deform as a result.

Furthermore, when the compact 20 with such a non-uniform densitydistribution is sintered, the resultant sintered body 30 should befurther deformed. This is because there is a correlation between therate of shrinkage of the compact 20 through the sintering process andthe density of the compact 20. That is to say, the shrinkage ratechanges with the density distribution. This problem is particularlynoticeable in a compact with a low density. Also, a thin compact isconsiderably affected by the distribution in the shrinkage rate, easilycracks or chips, and is likely deformed significantly.

On the other hand, it is known that a quality compact of a magneticpowder material can be obtained by a rubber pressing method. In thismethod, a magnetic powder material is loaded into a mold made of rubberand then immersed in a liquid medium such that a hydrostatic pressure isapplied to the magnetic powder material by way of the rubber mold.According to this rubber pressing method, a pressure can be appliedisotropically to the magnetic powder material. Thus, even if themagnetic powder material that has been loaded into the mold has anon-uniform density distribution, a compact with a uniform densitydistribution can still be obtained. Unfortunately, though, the rubberpressing method is a sort of hydrostatic pressure pressing process withvery low productivity and is hard to apply industrially.

Thus, to increase the low productivity of the rubber pressing process,Japanese Patent Gazette for Opposition No. 55-26601 proposes a paralleldie-pressing method in which a pre-molded rubber container is put into adie, filled with an alloy powder, and then pressure is applied theretoin the same direction as the magnetic field. In the pressing methoddisclosed in Japanese Patent Gazette for Opposition No. 55-26601,however, if a powder material with a low fill density, which has beenloaded by a natural loading technique, for example, is pressed, then theresultant compact likely cracks, chips or deforms.

To overcome such a problem, Japanese Laid-Open Publication No. 4-363010proposes a method of die-pressing a magnetic powder material that hasbeen loaded into a mold, having at least a rubber side surface and abottom, at a high density (which is at least 1.2 times as high as thenatural fill density). According to this method, however, the magneticpowder material 10 likely has a non-uniform fill density distributionwhile being loaded into such a rubber mold at a high density. Thus, theresultant compact 20 can have a uniform compact density as shown in FIG.13( c). But since the outer shape of the compact 20 reflects its filldensity distribution, it is difficult to obtain a compact in apredetermined shape. For that reason, to process a sintered body 30,obtained from such a compact 20, into the predetermined shape, all ofthe surfaces of the sintered body 30 must be machined. Also, this methodrequires high-density filling. Accordingly, when a magnetic powder witha small mean particle size and a narrow particle size distribution(e.g., a rare earth alloy powder obtained by a strip casting process) isused, the powder easily sticks together, thus causing a significantvariation in fill density. As a result, the problem becomes even morenoticeable.

As described above, none of the conventional techniques can compact apowder material with a non-uniform fill density distribution at a highproductivity with cracking, chipping or deformation of the compactminimized. In particular, none of those techniques can compact a powdermaterial with a low fill density (e.g., the rare earth alloy powdermaterial described above) at a high productivity.

In order to overcome the problems described above, an object of thepresent invention is to provide a powder compacting method and apparatusthat can make a compact with a uniform density distribution at a highproductivity even from a powder material with a non-uniform fill densitydistribution, and a method for producing a magnet by using them.

DISCLOSURE OF INVENTION

A powder compacting method according to the present invention includesthe steps of: providing a powder material; loading the powder materialinto a cavity; uniaxially pressing the powder material, which has beenloaded into the cavity, between two opposed press surfaces, therebyobtaining a compact, wherein at least one of the two press surfaces isdeformed elastically under a compacting pressure when contacting withthe powder material in the cavity; and unloading the compact from thecavity.

In one preferred embodiment, the at least one press surface is thesurface of a resin layer.

In another preferred embodiment, the resin layer has a Shore A hardnessof 25 to 95.

In another preferred embodiment, in the uniaxially pressing step, justone of the two press surfaces is deformed elastically under thecompacting pressure.

In another preferred embodiment, the loading step includes the step ofmeasuring the powder material with the cavity.

In another preferred embodiment, the loading step includes the step offilling the cavity with the powder material at a relative density of0.20 to 0.35.

In another preferred embodiment, the uniaxially pressing step includesthe step of uniaxially pressing the powder material to a volume that is0.5 to 0.65 time as large as the content volume of the cavity.

In another preferred embodiment, the compact satisfies D≦|S^(1/2)|/3,where D is the thickness (mm) of the compact as measured in a press axisdirection in the uniaxially pressing step and S is the area (mm²) ofeach of the two press surfaces.

An inventive method for producing a magnet includes the steps of:providing a powder material including a rare earth alloy powder; loadingthe powder material into a cavity; uniaxially pressing the powdermaterial, which has been loaded into the cavity, between two opposedpress surfaces, thereby obtaining a compact, wherein at least one of thetwo press surfaces is deformed elastically under a compacting pressurewhen contacting with the powder material in the cavity; and unloadingthe compact from the cavity.

In one preferred embodiment, the at least one press surface is thesurface of a resin layer.

In another preferred embodiment, the resin layer has a Shore A hardnessof 25 to 90.

In another preferred embodiment, in the uniaxially pressing step, justone of the two press surfaces is deformed elastically under thecompacting pressure.

In another preferred embodiment, the loading step includes the step ofmeasuring the powder material with the cavity.

In another preferred embodiment, the loading step includes the step offilling the cavity with the powder material at a relative density of0.20 to 0.35.

In another preferred embodiment, the uniaxially pressing step includesthe step of uniaxially pressing the powder material to a volume that is0.5 to 0.65 time as large as the content volume of the cavity.

In another preferred embodiment, the compact satisfies D≦|S^(1/2)|/3,where D is the thickness (mm) of the compact as measured in a press axisdirection in the uniaxially pressing step and S is the area (mm²) ofeach of the two press surfaces.

In another preferred embodiment, the method further includes the step ofaligning the rare earth alloy powder by applying a magnetic fieldthereto perpendicularly to the press axis direction during theuniaxially pressing step.

In another preferred embodiment, in the uniaxially pressing step, thepress axis direction is defined vertically, the two press surfacesconsist of an upper press surface and a lower press surface, the sidesurface of the cavity is defined by an inner surface of a die, and thebottom of the cavity is defined by the lower press surface.

In another preferred embodiment, the method further includes the stepsof: sintering the compact to obtain a sintered body; and finishing thesurface of the sintered body. The surface finishing step includes thestep of selectively grinding only a surface of the sintered body thatcontacted with the at least one press surface in the uniaxially pressingstep.

A powder compacting apparatus according to the present invention isprovided to uniaxially press a powder material that has been loaded intoa cavity. The apparatus includes: a die having an inner surface thatdefines the side surface of the cavity; a lower punch having a lowerpress surface that defines the bottom of the cavity; and an upper punchhaving an upper press surface that is opposed to the lower presssurface. In uniaxially pressing the powder material, which has beenloaded into the cavity, between the lower and upper press surfaces, atleast one of the lower and upper press surfaces is selectively deformedelastically under a compacting pressure among the inner surface, thelower press surface and the upper press surface that define the cavity.

In one preferred embodiment, the at least one press surface is thesurface of a resin layer.

In another preferred embodiment, the resin layer has a Shore A hardnessof 25 to 90.

In another preferred embodiment, just one of the lower and upper presssurfaces is deformed elastically under the compacting pressure.

In another preferred embodiment, the upper press surface is deformedelastically under the compacting pressure.

In another preferred embodiment, the upper press surface is the surfaceof a resin layer, and the upper punch includes a member for preventingthe resin layer from expanding in a horizontal direction, which isperpendicular to the press axis direction, under the compactingpressure.

In another preferred embodiment, the upper punch includes a concaveportion to receive the resin layer, and the side surface of the concaveportion prevents the resin layer from expanding in the horizontaldirection that is perpendicular to the press axis direction under thecompacting pressure.

In another preferred embodiment, the upper punch includes a resin layer,a portion of which changes its hardness in the press axis direction, andthe upper press surface is the surface of the resin layer.

In another preferred embodiment, the resin layer includes: a first resinlayer with a first hardness; and a second resin layer with a secondhardness that is lower than the first hardness, and the upper presssurface is the surface of the first resin layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing a powder compacting method according tothe present invention.

FIG. 2 schematically illustrates a cross-sectional structure of acompacting apparatus 100 according to the present invention, wherein:FIG. 2( a) illustrates a state in which a powder material 10 has justbeen loaded into a cavity; FIG. 2( b) illustrates a state in which acompacting pressure is applied thereto; and FIG. 2( c) illustrates astate in which a compact 20 is unloaded.

FIG. 3( a) is a perspective view schematically illustrating a powdercompacting apparatus 200 according to an embodiment of the presentinvention, and FIG. 3( b) is a schematic cross-sectional view of thepowder compacting apparatus 200.

FIG. 4 is an exploded perspective view schematically illustrating anupper punch 205 to be provided for the powder compacting apparatus 200.

FIG. 5 is an exploded perspective view schematically illustratinganother upper punch 405 for use in a powder compacting apparatusaccording to the present invention.

FIG. 6 schematically illustrates another upper punch 505 for use in apowder compacting apparatus according to the present invention, wherein:FIG. 6( a) is a cross-sectional view thereof; and FIG. 6( b) is a planview thereof.

FIG. 7 is a cross-sectional view schematically illustrating anotherupper punch 605 for use in a powder compacting apparatus according tothe present invention.

FIGS. 8( a) and 8(b) schematically illustrate a cross-sectionalstructure of the compacting apparatus that is performing a compactingprocess with the upper punch 605 shown in FIG. 7.

FIG. 9 is a cross-sectional view schematically illustrating anotherupper punch 705 for use in a powder compacting apparatus according tothe present invention.

FIG. 10( a) shows the results of estimated variations in the size ofsintered bodies obtained by the method for producing a magnet accordingto the invention along with the results that were estimated for sinteredbodies obtained by the conventional manufacturing process.

FIG. 10( b) is a schematic representation showing a method forestimating a size variation.

FIG. 11( a) shows the outer peripheral shape of a sintered body, whichwas obtained by using a resin layer with a Shore A hardness of 70.

FIG. 11( b) shows the outer peripheral shape of a sintered body, whichwas obtained by using an upper punch with no resin layer.

FIG. 12 schematically shows how to obtain the outer peripheral shapesshown in FIGS. 11( a) and 11(b).

FIGS. 13( a), 13(b) and 13(c) illustrate the features of various powdercompacting methods, wherein:

FIG. 13( b) shows a method that uses a normal die; and

FIG. 13( c) shows a rubber molding method.

FIG. 13( c) shows a method that uses a normal die.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of a powder compacting method and apparatusaccording to the present invention will be described with reference tothe accompanying drawings.

As shown in the flowchart of FIG. 1, a powder compacting methodaccording to an embodiment of the present invention includes: the stepS10 of providing a powder material; the step S20 of loading the powdermaterial into a cavity; the step S30 of uniaxially pressing the powdermaterial with at least one of two press surfaces deformed elasticallyunder a compacting pressure; and the step S40 of unloading a compactfrom the cavity. In the uniaxially pressing step S30, at least one oftwo opposed press surfaces, contacting with the powder material in thecavity, is deformed elastically under the compacting pressure.

That is to say, in the powder compacting method of the presentinvention, at least one of the two press surfaces (i.e., both or one ofthe two press surfaces) is deformed elastically under the compactingpressure, whereas at least the side surface of the cavity is notdeformed elastically under the compacting pressure but substantiallymaintains its original shape during the pressing process. Even if thepowder material in the cavity has a non-uniform fill densitydistribution, the non-uniform fill density distribution iscounterbalanced with the elastic deformation of the at least one presssurface, thereby applying a uniform pressure to the powder material. Forexample, as shown in FIG. 13( a), portions of the compact 20corresponding to low-fill-density portions of the powder material 10(i.e., as indicated by L in FIGS. 13( a) through 13(c)) are thinner thanthe other portion of the compact 20 corresponding to a high-fill-densityportion of the powder material 10 (as indicated by H in FIGS. 13( a)through 13(c)) as measured in a press axis direction (i.e.,perpendicularly to the press surfaces). That is to say, the non-uniformfill density distribution is transformed into a non-uniform thicknessdistribution of the compact 20, thus uniformizing the density of thecompact 20. According to the powder pressing method of the presentinvention, even a rare earth alloy powder, having a fill density that islow enough to have its orientation directions aligned under a magneticfield sufficiently, can be pressed into a compact with a uniform densitydistribution at a high productivity.

Furthermore, the surface to counterbalance the non-uniform fill densitydistribution is just the surface that has been in contact with theelastically deformed press surface. Thus, at most two opposed surfacesshould counterbalance that non-uniform fill density distribution.Accordingly, when just one of the two press surfaces is supposed to beelastically deformed, just one surface of the compact 20 hascounterbalanced the non-uniform fill density distribution and the othersurface of the compact 20 (i.e., the surface that has been in contactwith the non-elastically-deformed press surface) has a predeterminedshape (typically a flat shape) as shown in FIG. 13( a). This is becausethe other surface is defined by the side surface of the cavity and theother press surface that are substantially not deformed elasticallyunder the compacting pressure.

As described above, the compact 20 obtained by the powder compactingmethod of the present invention has a uniform density distribution, andtherefore, hardly chips, cracks, or deforms. Furthermore, the compact 20shrinks uniformly even during the sintering process, thus creatingalmost no chipping, cracking or deformation in the sintered body 30.Thus, high-quality compacts can be obtained at a high productivity. Inaddition, the powder pressing method of the present invention can alsoprocess a powder material with a relatively low fill density. Thus, themethod of the present invention can be used effectively to prepare acompact to be processed into a rare earth sintered magnet.

Furthermore, even though the compact shrinks through the sinteringprocess, the compact can still maintain a predetermined outer shape asdefined by the side surface of the cavity. Accordingly, it is just thesurface of the compact, which has been in contact with the elasticallydeformed press surface, that needs to be processed (e.g., grinded) by asubsequent finishing process. Thus, even in a situation where both ofthe two press surfaces are supposed to be deformed elastically, only thetwo opposed surfaces of the compact should be processed, but the sidesurface of the compact does not have to be processed. For example, if acompact with six sides is obtained by a conventional compacting method,all of those six sides need to be processed. In contrast, according tothe powder compacting method of the present invention, at most two sidesneed to be processed, thus increasing the throughput significantly.Furthermore, the processing margin (or grinding margin) can bedecreased, thus increasing the yield of the material, too. Inparticular, in the situation where just one of the two press surfaces issupposed to be deformed elastically as shown in FIG. 13( a), only onesurface needs to be subjected to the finishing process. As a result, theproductivity can be further increased.

The powder compacting method described above may be carried out with thepowder compacting apparatus 100 shown in FIG. 2, for example. FIGS. 2(a), 2(b) and 2(c) schematically illustrate a cross-sectional structureof the compacting apparatus 100. FIG. 2( a) illustrates a state in whicha powder material 10 has just been loaded into a cavity 112; FIG. 2( b)illustrates a state in which a compacting pressure is applied thereto;and FIG. 2( c) illustrates a state in which a compact 20 is unloaded.

The powder compacting apparatus 100 includes: a die 110 with an innersurface 110 a that defines the side surface of the cavity 112; a lowerpunch 130 with a lower press surface 130 a that defines the bottom ofthe cavity 112; and an upper punch 140 with an upper press surface 140 athat is opposed to the lower press surface 130 a. If necessary, thepowder compacting apparatus 100 may further include a magnetic fieldgenerating coil 206 to align the particles of a rare earth alloy powder,for example, under a magnetic field during the pressing process.

In this powder compacting apparatus 100, only the upper press surface140 a, not the inner surface 110 a or the lower press surface 130 a, isdeformed elastically under a compacting pressure while the powdermaterial 10 is being pressed uniaxially. The lower and upper punches 130and 140 can be freely inserted into, and removed from, the opening 112of the die 110 with a predetermined clearance allowed with respect tothe inner surface 110 a. The opening and the cavity are identifiedherein by the same reference numeral.

In the following example, only the upper press surface 140 a is deformedelastically. Naturally, only the lower press surface 130 a may bedeformed elastically. Alternatively, both the lower and upper presssurfaces 130 a and 140 a may be deformed elastically. However, just oneof the lower and upper press surfaces 130 a and 140 a is preferablydeformed elastically because the subsequent finishing process can besimplified in that case. That is to say, to obtain a compact in apredetermined shape by the subsequent surface finishing process, one ofthe two surfaces may be used as a reference plane for the surface finishand only the other surface may be processed (e.g., grinded).

The powder compacting apparatus 100 may be the same as a knowndie-pressing apparatus except that just the upper press surface 140 a isselectively deformed elastically under a compacting pressure to beapplied in the uniaxial pressing process among the surfaces that definethe cavity 112 and the upper press surface 140 a (i.e., the surfacesthat contact with the powder material during the pressing process). Thedie 110, the lower punch 130 and the base 144 of the upper punch 140 maybe made of a metal (e.g., SUS 304). Also, the die 110 and the lower andupper punches 130 and 140 may be driven hydraulically.

The press surface 140 a to be deformed elastically under the compactingpressure may be formed by providing a pressure medium layer 142 havingan appropriate mechanical property (which is well represented by itsShore hardness) on the surface of the metallic base 144. The pressuremedium layer 142 does not always have to be a solid but may also be aliquid which is hermetically packed in an appropriate bag. Nevertheless,it is still convenient to use a solid layer. For example, a resin layeris preferably used as the pressure medium layer 142. A resin having aShore A hardness of 25 to 90 is preferably used as a material for theresin layer. A resin layer with a Shore A hardness of 60 to 85 isparticularly preferred. Specifically, the resin layer is preferably madeof a urethane resin including urethane rubber.

Hereinafter, the operation of the powder compacting apparatus 100 andthe powder compacting method of the present invention will be describedwith reference to FIGS. 2( a) through 2(c).

First, as shown in FIG. 2( a), the powder material 10 is loaded into thecavity 112. The powder may be loaded by any of various known methods.However, the powder compacting apparatus and method of the presentinvention can be used effectively to compact the powder material 10 thathas been loaded there at a low fill density (or to form a thin compactamong other things). Thus, a loading method to be preferably adopted forthat purpose will be described. The powder material to be used is notparticularly limited. This is because according to the powder compactingmethod of the present invention, a quality compact can be obtained evenfrom a powder material with particularly poor flowability (i.e.,loadability and/or compactability).

A material including the rare earth alloy powder (e.g., an R—Fe—B basedalloy powder) prepared by the strip casting process mentioned above maybe used as the powder material. To increase the flowability (i.e.,loadability and compactability), a powder material, in which a lubricanthas been added in a predetermined amount (e.g., at most 0.12 wt % ofaliphatic ester) to the surface of a rare earth alloy powder with apredetermined particle size (of 2 μm to 6 μm, for example), is typicallyused. Optionally, it is not impossible to use a material obtained bygranulating a rare earth alloy powder with a lubricant or a binder.However, such a material is not preferred because a higher magneticfield is needed to decompose the granulated particles into primaryparticles and thereby align those rare earth alloy powder particlesunder a magnetic field. Also, if carbon, included in the lubricant orbinder to be added to the rare earth alloy powder, is left in thesintered body, then that carbon may deteriorate the magnetic properties.For that reason, the amount of those additives is preferably as small aspossible, generally speaking. It is one of the reasons why a rare earthalloy powder is hard to compact into a desired shape that the amount ofthe additive such as the lubricant is limited in this manner.

The process step of loading the powder material may be carried out byusing either a sieve or a feeder box as disclosed in Japanese PatentPublication for Opposition No. 59-40560, Japanese Laid-Open PublicationNo. 10-58198, Japanese Utility Model Publication No. 63-110521 andJapanese Laid-Open Publication No. 2000-248301. By adopting such aloading technique, the powder can be loaded at a fill density that islow enough to have its orientation directions aligned under the magneticfield.

Particularly when a powder material with poor flowability (or filldensity) such as a rare earth alloy powder prepared by a strip castingprocess should be loaded, the method disclosed in Japanese Laid-OpenPublication No. 2000-248301, which was filed by the applicant of thepresent application, is preferably used. In that method, a feeder boxwith an opening at the bottom is positioned over a cavity and a barmember is horizontally reciprocated on the bottom of the feeder box,thereby feeding an alloy powder material from the feeder box into thecavity. In this manner, the alloy powder can be loaded from the feederbox into the cavity sequentially (i.e., beginning with a part of thepowder near the bottom) at an equal pressure. As a result, the cavitycan be filled with the powder at a relatively uniform density withoutcausing any lumps or bridges.

To obtain a thin compact, a powder material in an amount correspondingto the inner volume of the cavity is preferably measured with the cavityin the loading process step. For example, if the bar member isreciprocated over the cavity as in the method described above, then thecavity can be filled with the powder material that has been fed theretowith the excessive portion thereof sliced off. Thus, the cavity can befilled with the predetermined amount of powder material relativelyuniformly. It should be noted that if the powder material is loaded bysuch a method, then a non-uniform loading weight (or fill density)distribution is likely formed on the surface of the powder materialloaded (i.e., on the upper surface of the cavity) in the direction inwhich the bar member is going back and forth. Accordingly, if such apowder material is uniaxially pressed by using a conventionaldie-pressing apparatus in which no press surfaces are deformedelastically under the compacting pressure, then the resultant compactshould have a non-uniform density distribution and may chip, crack ordeform. However, according to the powder pressing method of the presentinvention, a compact with a uniform density distribution can beobtained. In forming a thin compact, in particular, the non-uniform filldensity distribution near the surface of the powder material should havesignificant effects. Thus, the present invention is particularlyeffective in such a situation.

According to any of the various loading techniques described above, thepowder material can be loaded into the cavity at a relative density of0.20 to 0.35. As used herein, the “relative density” means the ratio ofthe fill density of the powder material to the true density thereof. Ifthe powder material is measured with the cavity, the fill densitythereof is given by dividing the mass of the powder material in thecavity by the inner volume of the cavity. The powder material that hasbeen loaded at such a relative density can have its orientationdirections sufficiently aligned under a magnetic field even if thepowder material is a rare earth alloy powder prepared by a strip castingprocess.

Next, as shown in FIG. 2( b), by lowering the upper punch 140, forexample, the powder material 10 that has been loaded into the cavity 112is uniaxially pressed between the lower and upper press surfaces 130 aand 140 a. The powder material that has been loaded typically at arelative density of 0.20 to 0.35 is uniaxially pressed in this uniaxialpressing step. As a result, a compact having a relative density (i.e.,the ratio of the green density of the compact to the true densitythereof) of 0.5 to 0.7 can be obtained. The compacting pressure may bein the range of 50 kgf/cm² to 5,000 kgf/cm² (i.e., in the range of 4.9MPa to 490 MPa). For example, if a rare earth alloy powder (e.g., anR—Fe—B based alloy powder) prepared by a strip casting process is used,then the compacting pressure preferably falls within the range of 500kgf/cm² to 1,000 kgf/cm² (i.e., the range of 49 MPa to 98 MPa). In thatcase, a compact, of which the density is about 52% to about 62% of thetrue density, can be obtained.

Optionally, before the uniaxial pressing process is started, a lubricant(which may be the same as that applied to the surface of the rare earthalloy powder) may be sprayed onto the powder material 10 in the cavity112 and onto the surface of the upper punch 140. A urethane resin isparticularly preferred as a material for the resin layer 142 because theurethane resin exhibits an appropriate Shore hardness, excellentabrasion resistance and good resistance to the lubricant.

In this uniaxial pressing process, the upper press surface 140 a, whichmay be the surface of the resin layer 142, for example, is elasticallydeformed due to a non-uniform pressure distribution resulting from thenon-uniform fill density distribution of the powder material 10. On theother hand, the lower press surface 130 a and the inner surface 110 a ofthe opening 112 of the die 110, which may be made of SUS, for example,are not elastically deformed substantially under the compacting pressureeven though those surfaces contact with the powder material 10.Accordingly, the bottom and the side surface of the powder material 10being compacted maintain their predetermined shapes. But just itssurface in contact with the upper press surface 140 a is deformed insuch a manner as to counterbalance the non-uniform density distribution.Consequently, the resultant compact 20 has a uniform densitydistribution and chipping, cracking or deformation thereof can beminimized.

In particular, even if the resultant compact is thin enough to satisfyD≦|S^(1/2)|/3 (where D is the thickness (mm) of the compact as measuredin the press axis direction and S is the area (mm²) of each presssurface), chipping or cracking thereof can still be reducedsufficiently. The thickness of the resin layer 142 is preferably at mosttwice greater than the thickness D (mm) of the compact. The thickness ofthe resin layer 142 should not be more than twice greater than thethickness D (mm) of the compact because the pressure cannot betransmitted efficiently in that case. The thickness of the resin layer142 is not particularly limited as long as the resin layer 142 cancounterbalance the non-uniform fill density distribution. However, thethickness of the resin layer 142 is at least one-third of the thicknessD (mm) of the compact. This is because an excessively thin resin layer142 may not function as a pressure medium effectively enough.

It should be noted that to align the rare earth alloy powder particlesunder a magnetic field, the magnetic field is applied externally duringthe uniaxial pressing process. For example, a magnetic field of about0.8 MA/m to about 1.3 MA/m may be applied perpendicularly to theuniaxial pressing direction. On the application of such a high aligningmagnetic field, if the die used has lower saturation magnetization thanthe powder loaded, then the powder is attracted toward both ends of thecavity in the aligning direction (i.e., the side surface thereof) duringthe aligning process. In this manner, the fill density of the powder mayfurther vary upon the application of the aligning magnetic field. Evenso, a compact having a uniform density can also be obtained according tothe present invention.

Next, the resultant compact 20 is unloaded from the cavity. This processstep may be carried out by any of various known techniques. However, acompact with a relatively low density (of which the green density is 50%to 70% of the true density thereof), made of a material with poorflowability such as a rare earth alloy powder material prepared by astrip casting process, is brittle. Accordingly, such a compact ispreferably unloaded from the cavity 112 by a hold-down technique, inwhich the die 110 is lowered with a certain pressure (e.g., about 1% toabout 20% of the compacting pressure) maintained between the upper andlower press surfaces 130 a and 140 a such that the surface of thecompact 20 that has been in contact with the inner surface 110 a of theopening 112 is exposed as shown in FIG. 2( c). In that case, only theupper press surface 140 a is preferably deformed elastically. This isbecause if the press surface to be deformed elastically is the surfaceof the resin layer, the surface of the resin layer will not adhere soclosely to the compact as the surface of the metal. Thus, it is possibleto avoid a situation where the compact adheres so strongly to the resinlayer as to be lifted by the resin layer over the die. That is to say,the compact will not drop and chip or crack. Also, if the lower presssurface 130 a is deformed elastically, then the bottom of the compact 20will have some unevenness. In that case, portions of the bottom of thecompact 20 will be located at a lower level than the upper surface ofthe die 110. Thus, the compact 20 easily chips or cracks while beingunloaded from the cavity 112.

Also, if the upper press surface 140 a is defined by the surface of theresin layer 142, the resin layer 142 that has left the cavity 112 isalso expanded under the compacting pressure in a horizontal directionthat is perpendicular to the press axis direction. Due to thisdeformation of the resin layer 142, the compact 20 may chip or crackaround its periphery. To minimize such chipping or cracking, a member toprevent the resin layer 142 from being expanded in the horizontaldirection (i.e., perpendicularly to the press axis direction) ispreferably provided. For example, the resin layer 142 is preferablyfitted in a concave portion, which is provided in the base 144, suchthat the deformation of the surface of the resin layer 142(corresponding to the press surface 140 a) in the directionperpendicular to the press axis direction is minimized by the wall ofthe concave portion. Instead, the resin layer 142 is preferably allowedto be deformed only in the press axis direction inside of the concaveportion.

Hereinafter, an embodiment of a method for producing a sintered magnetof an R—Fe—B based alloy powder, prepared by a strip casting process,will be described.

First, an alloy flake, having a composition including 30 wt % of Nd, 1.0wt % of B, 1.2 wt % of Dy, 0.2 wt % of Al, 0.9 wt % of Co, and Fe andinevitable impurities as the balance, is prepared by a strip castingprocess (see U.S. Pat. No. 5,383,978, for example). More specifically,an alloy that has been prepared by a known method so as to have acomposition including 30 wt % of Nd, 1.0 wt % of B, 1.2 wt % of Dy, 0.2wt % of Al, 0.9 wt % of Co, and Fe and inevitable impurities as thebalance is melted by a high-frequency melting process to obtain a melt.As the rare earth alloy, not only an alloy having such a composition butalso an alloy having the composition disclosed in U.S. Pat. No.4,770,723 or 4,792,368 may be used effectively.

The melt of this rare earth alloy is heated to, and maintained at, 1350°C. and then rapidly cooled on a single roller at a peripheral velocityof about 1 m/sec, a cooling rate of 500° C./min and a supercooling rateof 200° C., thereby obtaining an alloy flake with a thickness of 0.3 mm.This alloy flake is embrittled by being subjected to a hydrogenocclusion process to obtain an alloy coarse powder. Then, the alloycoarse powder is finely pulverized by a jet mill machine within anitrogen gas atmosphere, thereby obtaining an alloy powder with a meanparticle size of 3.5 μm. This alloy powder has a true density of 7.5g/cm³. This fine pulverization process is preferably carried out by theapparatus and method disclosed in Japanese Patent Application No.11-62848. The finely pulverized powder of the alloy, prepared by a rapidcooling process such as a strip casting process (at a cooling rate of10² to 10⁴° C./sec) in this manner, has a narrow particle sizedistribution and exhibits poor compactability but can still be usedeffectively as a material for a magnet exhibiting good magneticproperties.

Next, to improve the flowability (i.e., loadability and/orcompactability) of the alloy powder obtained in this manner, the surfaceof the alloy powder is coated with a lubricant. For example, aliphaticester may be used as the lubricant and diluted with a petroleum solvent.The mixture may be added at 0.5 wt % to 5.0 wt % (on the lubricantbasis) to the resultant alloy powder in a rocking mixer, thereby coatingthe surface of the alloy powder with the lubricant. Methyl caproate maybe used as the aliphatic ester and isoparaffin may be used as thepetroleum solvent. The weight ratio of methyl caproate to isoparaffinmay be 1:9.

However, the lubricant is not limited to any particular type. Forexample, any aliphatic ester diluted with any solvent may be used.Examples of preferred aliphatic esters include not just methyl caproatebut also methyl caprylate, methyl laurate and methyl laurylate. As thesolvent, petroleum solvents such as isoparaffin and naphthene solventsmay be used. The aliphatic ester and the solvent may be mixed at aweight ratio of 1:20 to 1:1. Instead of, or in addition to, the liquidlubricant, a solid lubricant such as zinc stearate may also be used.When a liquid lubricant is used, no solvent may be used.

The weight of the lubricant to be added may be determined appropriately.However, to improve the compactability and magnetic properties, thepowder material to be compacted preferably includes no greater than 0.12wt % of lubricant to the overall weight of the alloy powder.

Next, a uniaxial pressing process is carried out using a powdercompacting apparatus 200 according to an embodiment of the presentinvention as shown in FIGS. 3( a) and 3(b). FIG. 3( a) is a schematicperspective view of the powder compacting apparatus 200 and FIG. 3( b)is a schematic cross-sectional view of the powder compacting apparatus200.

The powder compacting apparatus 200 includes a powder material feedingmechanism 300. A die set 202 is disposed adjacent to a base plate 201. Adie 202 a is fitted in the die set 202 and is provided with an opening(die hole) 202 b that runs through the die 202 a vertically. A lowerpunch 203 is provided under this die hole 202 b so as to freely go upand down inside the die hole 202 b. A cavity 204 having an arbitraryinner volume is defined by the inner surface 204 a of this die hole 202b and the press surface 203 a of the lower punch 203. In the illustratedexample, a thin rectangular cavity 204 is defined. The cavity 204 mayhave a longer-side length of 80 mm, a shorter-side length of 52.2 mm anda depth of 16 mm, for example.

After an alloy powder has been fed into the cavity 204 by using thepowder material feeding mechanism 300, an upper punch 205 is introducedinto the cavity 204, and the alloy powder material is uniaxially pressedbetween the press surface 205 a of the upper punch 205 and the presssurface 203 a of the lower punch 203, thereby forming a compact of thealloy powder material. A pair of magnetic field generating coils 206 isprovided on both sides of the die 202 a to apply a magnetic fieldperpendicularly to the uniaxial pressing direction and parallelly to thelonger-side direction of the cavity 204 as indicated by the arrow B inFIG. 3( a).

The die 202 a, the lower punch 203 and the base 214 of the upper punch205 are made of a stainless steel (e.g., SUS 304). The resin layer 212of the upper punch 205 is made of a urethane resin with a Shore Ahardness of 75 to 80. As already described with reference to FIGS. 2( a)through 2(c), this resin layer 212 is deformed elastically under acompacting pressure in accordance with the fill density distribution,thereby obtaining a compact with a uniform density.

In this example, the loading technique that uses the powder materialfeeding mechanism 300 disclosed in Japanese Laid-Open Publication No.2000-248301 is used. However, the present invention is in no way limitedto this loading technique, but the powder material may be loaded by anyof various other methods.

The powder material feeding mechanism 300 includes a feeder box 310 onthe base plate 201. This feeder box 310 is driven by the cylinder rod311 a of an air cylinder 311 so as to go back and forth between aposition over the die 202 a and a standby position. A supplier 330 forsupplying the feeder box 310 with the rare earth alloy powder isprovided near the standby position of the feeder box 310.

A feeder cup 331 is placed on the balance 332 of the supplier 330 suchthat the alloy powder material is dropped little by little into thefeeder cup 331 by a vibrating trough 333. This measuring operation iscarried out while the feeder box 310 is gone away to the die 202 a. Whenthe feeder box 310 is back to the standby position, the feeder box 310is supplied again by a robot 334. The weight of the alloy powdermaterial to be put into the feeder cup 331 is set equal to the weight ofthe alloy powder material that is removed from the feeder box 310 by asingle pressing operation. That is to say, the weight of the alloypowder material in the feeder box 310 is always kept constant. Since aconstant amount of alloy powder material is always stored in the feederbox 310, a constant pressure is applied to the powder material that isdropping into the cavity 204 because of the pull of gravity. As aresult, a constant weight of alloy powder material is loaded into thecavity 204.

A shaker 320, provided inside the feeder box 310, is secured to twosupporting rods 312 by way of coupling bars 322 a. The two supportingrods 312 extend parallelly through a pair of sidewalls 310 a, whichfaces the direction in which the feeder box 310 is reciprocated. Bothends of each of these two supporting rods 312 are screwed up withcoupling members 313. A second air cylinder 315 is secured to a fixingmember 314, which is attached to the outer surface of the sidewall 310 aon the right-hand side in FIG. 3( b). The cylinder shaft 315 a of theair cylinder 315 is secured to the coupling member 313 on the right-handside. By supplying the air through air supply tubes 315 b to both endsof the air cylinder 315, the cylinder shaft 315 a can be reciprocated,thereby reciprocating the shaker 320.

Upper and lower pairs of bar members 321 are provided for the shaker 320so as to extend parallelly to the horizontal direction, i.e., thedirection that is perpendicular to the longer-side direction of thecavity 204. These bar members 321 may be cylindrical bars, each having acircular cross section with a diameter of 0.3 mm to 7 mm, for example.The upper and lower pairs of bar members 321 are combined together by asupporting member 322 into a frame shape. By reciprocating the cylindershaft 315 a of the air cylinder 315, these bar members 321 can also goback and forth horizontally inside the feeder box 310. The pitch of thebar members 321 as measured in the direction in which they move is setsubstantially equal to the longer-side length of the cavity 204. Thelower pair of bar members 321 is provided such that the lower endthereof is located 0.2 mm to 5 mm over the surface of the diesurrounding the cavity 204. Also, the bar members 321, as well as thesupporting member 322, are made of a stainless steel (e.g., SUS 304).

An N₂ gas supply pipe 323 to supply an inert gas into the feeder box 310is connected to a point above the center of the right-hand-side sidewall310 a of the feeder box 310. The inert gas is supplied at a higherpressure than the atmospheric pressure so as to maintain an inert gasatmosphere inside the feeder box 310. Accordingly, even if some frictionis created between the reciprocating shaker 320 and the alloy powdermaterial, no firing should occur. Also, even when the feeder box 310moves with the alloy powder material sandwiched between the bottom ofthe feeder box 310 and the base plate 201, no firing should be caused bythe friction, either. Furthermore, even if some friction is createdbetween the powder particles in the feeder box 310 due to the movementof the feeder box 310, no firing should occur, either.

A lid 310 d is provided so as to close the powder storage 310A of thefeeder box 310 airtight. In supplying the alloy powder material, thislid 310 d shifts rightward in FIG. 3( a) to make the upper surface ofthe powder storage 310A open. For that purpose, a third air cylinder 317to drive and open the lid 310 d is provided for the sidewall 310 b,which is located on the front side in FIG. 3( a). The air cylinder 317and the lid 310 d are coupled together via a fixing member 318 andscrewed up with each other. To maintain an inert gas atmosphere, thislid 310 d is normally located over the powder storage 310A of the feederbox 310. Only when the powder is supplied, the lid 310 d movesrightward. It should be noted that a guide means (not shown) is providedon the other side of the lid 310 d, which is opposed to the third aircylinder 317, to allow the lid 310 d being driven and opened by thethird air cylinder 317 to move smoothly. By supplying the air throughair supply tubes 317 b to both ends of the air cylinder 317, thecylinder shaft (not shown) is driven, thereby opening or closing the lid310 d.

Also, plate members 319, made of a fluorine resin and having a thicknessof 5 mm, are screwed up with the bottom of the feeder box 310 such thatno alloy powder material eats into the feeder box 310 or the base plate1 (or the die set 202) in the gap between them by sliding the feeder box310 on the base plate 201 with the fluorine resin plate members 319interposed between them.

Hereinafter, it will be described how to perform the powder feedingoperation with this powder material feeding mechanism 300.

First, an inert gas is introduced through the N₂ gas supply pipe 323into the powder storage 310A of the feeder box 310. In such a state, thelid 310 d of the feeder box 310 is opened, thereby feeding thepredetermined amount of alloy powder material, which has been measuredby the robot 334 with the feeder cup 331, into the powder storage 310A.Once the alloy powder material has been supplied, the lid 310 d isclosed to maintain the inert gas atmosphere inside the powder storage310A. It should be noted that the inert gas is always supplied into thepowder storage 310A, not just while the feeder box 310 is moving overthe cavity 204, thereby minimizing the potential alloy powder materialfiring. Alternatively, Ar or He may also be used as the inert gas.

In such a state, the air cylinder 311 is started to drive the feeder box310 toward the cavity 204 of the die 202. In this case, if the feederbox 310 is driven with the bar members 321 located on the front end ofthe direction of movement, then the alloy powder material on the frontend of the movement direction will not be shifted toward the rear end ofthe movement direction as the feeder box 310 moves. Thus, the alloypowder material can be transported to the cavity 204 with thenon-uniformity minimized.

After the feeder box 310 is positioned over the cavity 204 in thismanner, the alloy powder material in the feeder box 310 is loaded downinto the cavity 204 within the inert gas atmosphere while horizontallyreciprocating the bar members 321 five to fifteen times, for example,inside the feeder box 310. The final rest positions, at which the barmembers 321 should be eventually located after the horizontal movement,are defined such that all of those bar members 321 are kept off theopening 204 a of the cavity 204. In this manner, the alloy powdermaterial can be supplied into the cavity 204 at a relatively uniformfill density without running the risk of firing, for example. It shouldbe noted, however, that the bar members 321 slice off the excessivealloy powder material that has overflowed from the cavity 204. Thus,traces (i.e., non-uniform distribution of loading weight or filldensity) are formed on the surface of the alloy powder material that hasbeen loaded into the cavity 204 in the direction in which the barmembers 321 have moved (i.e., the same as the direction in which thefeeder box 310 has moved). To minimize such a non-uniform distribution,the movement direction of the bar members 321 is preferably theshorter-side direction of the cavity 204.

Next, after the alloy powder material has been loaded and supplied intothe cavity 204, the bar members 321 are shifted toward the front end ofthe backward direction of the feeder box 310, thereby preventing thealloy powder material on the front end of the (backward) movementdirection from going back toward the rear end of the (backward) movementdirection. Thereafter, the feeder box 310 is driven backward and theupper punch 205 is lowered, thereby compacting the alloy powder materialin the cavity 204. In the meantime, the alloy powder material is newlysupplied into the feeder box 310. The pressing process will be describedin detail later.

By repeatedly performing these operations, the alloy powder material canbe uniaxially pressed continuously. In the example described above, justone cavity 204 is provided. However, the same process is applicable foruse even in a situation where multiple cavities 204 are provided. Inthat case, however, multiple bar members 321 are preferably provided ata pitch substantially corresponding to the pitch of the multiplecavities 204 as measured in the direction in which the feeder box 310moves.

In this manner, the alloy powder material can be measured with thecavity 204, and can be loaded into the cavity 204, to the amountcorresponding to the inner volume of the cavity 204. In this case, thefill density may be 2.2 g/cm³ to 2.3 g/cm³ and the filling ratio (i.e.,the ratio of the relative density to the true density) may be 0.29 to0.31.

Hereinafter, the uniaxial pressing process will be described.

In this preferred embodiment, by lowering the upper punch 205, thepowder material is uniaxially pressed between the upper and lower presssurfaces 205 a and 203 a. In this uniaxial pressing process, only theupper press surface 205 a is deformed elastically but the inner surface204 a of the die hole 202 b and the lower press surface 203 a aresubstantially not deformed elastically although all of these surfacescontact with the powder material.

The structure of the upper punch 205 will be described with reference toFIG. 4. FIG. 4 is an exploded perspective view of the upper punch 205.

The upper punch 205 includes the resin layer 212 and the base 214. Thesurface of the resin layer 212 defines the upper press surface 205 a.The base 214 is made of a stainless steel (e.g., SUS 304) while theresin layer 212 is made of a urethane resin with a Shore A hardness(according to ISO 868) of 75 to 80. A thermosetting urethane resin Ureolproduced by Nihon Ciba Geigy Limited may be used as the urethane resin.

The resin layer 212 includes a flat plate portion 212 a and anchorportions 212 b. The anchor portions 212 b are fitted into the holes 214c of the base 214 and may be secured to the base 214 with an adhesive ifnecessary. The anchor portions 212 b are preferably provided to achievea sufficient strength but may be omitted as well. The base 214 shown inFIG. 4 includes a body 214 a and an end portion 214 b having a surfaceto which the resin layer 212 is secured. Optionally, these portions maybe combined together.

The thickness of the resin layer 212 (i.e., the thickness of the flatplate portion 212 a) may be about 5 mm, for example. Each of the anchorportions 212 b may have a cylindrical shape with a diameter of about 5mm and a height of about 10 mm, for example. The flat plate portion 212a and the anchor portions 212 b are integrated together. Such a resinlayer 212 may be made of the thermosetting urethane resin describedabove by a casting process, for example.

This resin layer 212 has a Shore A hardness of 75 to 80. Accordingly, ifthe alloy powder material is pressed at a pressure of 660 kgf/cm² (i.e.,64.7 MPa), the resin layer 212 is deformed elastically in accordancewith the non-uniform fill density distribution of the alloy powdermaterial, thereby applying a uniform pressure onto the alloy powdermaterial. By applying the pressure for a predetermined amount of time, acompact with a density of 4.1 g/cm³ can be obtained. That is to say, thepowder can be compacted to about 50% of the inner volume of the cavity204 as a result of this uniaxial pressing process. This uniaxialpressing process may be controlled by a normal technique.

After the uniaxial pressing process is finished, the die 202 is loweredwith the compacting pressure maintained at 33 kgf/cm² (i.e., 3.24 MPa),thereby exposing the side surface of the compact. Thereafter, the upperpunch 205 is raised to unload the compact. In this case, the adhesion ofthe resin layer 212 (i.e., the upper press surface 205 a) to the compactis weaker than that of the stainless steel plane (i.e., the lower presssurface 203 a) to the compact. Thus, the compact never goes up with theupper punch 205 and never drops or chips.

In the hold down technique in which the compact is extracted from thedie hole while being sandwiched between the upper and lower punches,when the upper punch 205 leaves the cavity 204, the compact is releasedfrom the pressure that has been applied thereto by the inner surface 204a of the cavity 204. As a result, due to the springback force of thepressed compact, the resin layer 212 expands in the horizontaldirection, or perpendicularly to the press axis direction. Thisexpansion may pull the surface of the compact that is in contact withthe resin layer 212, thus sometimes causing chipping around theperiphery of the compact.

However, by using the upper punch 405 shown in FIG. 5 instead of theupper punch 205 shown in FIG. 4, such chipping resulting from thedeformation of the resin layer can be minimized.

The upper punch 405 includes a resin layer 412 and a base 414. Thesurface of the resin layer 412 defines the upper press surface 205 a.The base 414 is made of a stainless steel (e.g., SUS 304) while theresin layer 412 is made of a urethane resin with a Shore A hardness of75 to 80.

The resin layer 412 includes a flat plate portion 412 a and anchorportions 412 b. The side surface 412 c of the flat plate portion 412 adefines a taper angle of about 60 degrees with respect to the presssurface 405 a.

The base 414 includes a concave portion 414 d to receive the resin layer412 therein. The anchor portions 412 b of the resin layer 412 are fittedinto the holes 414 c of the base 414 and may be secured to the base 414with an adhesive if necessary. The base 414 shown in FIG. 5 includes abody 414 a and an end portion 414 b having a surface to which the resinlayer 412 is secured. Optionally, these portions may be combinedtogether.

If the resin layer 412 is introduced into the concave portion 414 d ofthe base 414 in this manner, then the side surface of the concaveportion 414 d can prevent the resin layer 412 from expanding in thehorizontal direction, or perpendicularly to the press axis direction,due to the springback force of the pressed compact in the hold downprocess.

Alternatively, the upper punch 505 as schematically illustrated in FIGS.6( a) and 6(b) may also be used. The upper punch 505 includes a base514, a resin layer 512 and a deformation minimizing portion 515, whichis provided so as to substantially surround the resin layer 512 (exceptthe press surface 505 a, though). The deformation minimizing portion 515is made of a material (e.g., a resin or a metal) that has a higherelastic modulus than that of the material of the resin layer 512. Thus,the deformation minimizing portion 515 minimizes the resin layer 512from expanding in the horizontal direction, or perpendicularly to thepress axis direction, due to the springback force of the as-pressedcompact.

As another alternative, the upper punch 605 as schematically illustratedin FIG. 7 may also be used. The upper punch 605 includes a base 614 madeof a stainless steel (e.g., SUS 304) and a resin layer 612 having amultilayer structure.

The resin layer 612 includes a first resin layer 612 a and a secondresin layer 612 b, which are stacked one upon the other on the base 614and which have mutually different hardness values. Specifically, thehardness of the first resin layer 612 a is higher than that of thesecond resin layer 612 b. Thus, the first resin layer 612 a will bereferred to herein as a “hard resin layer” 612 a and the second resinlayer 612 b will be referred to herein as a “soft resin layer” 612 b.The hard resin layer 612 a is made of a urethane resin with a Shore Ahardness of 70 to 90, while the soft resin layer 612 b is made of aurethane resin with a Shore A hardness of 25 to 60. As can be seen fromFIG. 7, the surface of the hard resin layer 612 a defines the upperpress surface 605 a in this resin layer 612.

As described above, while the upper punch is extracted from the cavity,the resin layer may expand in the horizontal direction, orperpendicularly to the press axis direction. To minimize this expansion,the upper punches 405 and 505 shown in FIGS. 5 and 6 include ahigh-hardness deformation minimizing portion to receive the periphery ofthe resin layer. However, when any of these arrangements is adopted, theperipheral and central regions of the press surface have mutuallydifferent elastic moduli in the press axis direction. This may not bepreferable to apply a uniform pressure to the alloy powder that has beenloaded into the cavity.

In contrast, when the upper punch 605 including the resin layer 612 witha multilayer structure as shown in FIG. 7 is used, the elastic modulusof the resin layer 612 can be uniformized over the entire press surface605 a. Thus, the resultant compact can have a more uniform density.

Also, in the upper punch 605, the upper press surface 605 a to contactwith the compact is defined by the surface of the hard resin layer 612 aand the soft resin layer 612 b is provided between the hard resin layer612 a and the base 614. By adopting such an arrangement, even if theresin layer 612 expands in the horizontal direction, or perpendicularlyto the press axis direction, while the upper punch 605 is beingextracted from the cavity, the surface of the resin layer 612 (i.e., thesurface of the hard resin layer 612 a) is not damaged due to thatexpansion or the compact will not chip.

FIGS. 8( a) and 8(b) show how the powder material 10 may be compactedwith the upper punch 605. As shown in FIG. 8( a), when a pressure isapplied to the powder material 10 in the cavity, the soft resin layer612 b is deformed elastically in accordance with the variation in thefill density of the powder. However, since the hard resin layer 612 a isprovided, the soft resin layer 612 b will not be deformed excessively.Accordingly, no excessive unevenness will be formed on the press surfaceto contact with the compact (i.e., on the surface of the hard resinlayer 612 a).

It should be noted that the shapes of the press surfaces during thecompaction process may be controlled by adjusting the ratio of thethickness of the hard resin layer 612 a to that of the soft resin layer612 b, for example. If there is not so significant variation in the filldensity of the powder, then the hard resin layer 612 a may have arelatively small thickness, for instance.

While the upper punch 605 is extracted from the cavity by lowering thedie 110 after the compaction process has been carried out in thismanner, the resin layers 612 a and 612 b are going to extend in thehorizontal direction, or perpendicularly to the press axis direction,due to the springback force of the compact or the expansion of the resinlayers themselves as shown in FIG. 8( b).

However, the press surfaces, which are in contact with the compact, havenot been deformed excessively. Accordingly, even when the expandingforce described above is acting on the press surfaces, the compact orthe resin layers will not be damaged. In addition, the compact can alsobe removed from the punches advantageously.

Also, since the soft resin layer 612 b is provided, the expanding forceof the hard resin layer 612 a may be relaxed. During the compactionprocess, the soft resin layer 612 b is deformed to a great degree butthe hard resin layer 612 a is deformed to a small degree. Accordingly,the expansion of the hard resin layer itself may be reduced. Thus, thestress on the surface of the hard resin layer 612 a (i.e., the presssurface) can be reduced and cracking of that surface can be minimized.As a result, it is possible to prevent the compact from chipping.

In the example described above, the two resin layers 612 a and 612 b areused. Alternatively, the resin layer 612 with the multilayer structuremay consist of three or more resin layers with mutually differenthardness values. As another alternative, an upper punch 705 having aresin layer 712 of which the hardness changes gradually in the pressaxis direction may also be used as shown in FIG. 9. In that case, thehardness of the resin layer 712 preferably decreases gradually from thesurface 705 a of the resin layer 712 toward the junction plane 705 bbetween the resin layer 712 and the base 714.

Also, when the compaction process is carried out with an upper punchincluding a resin layer as described above, the powder material may becompacted after an easily deformable thin cloth member (i.e., a memberthat can change its shape with the elastic deformation of the resinlayer) has been sandwiched between the surface of the resin layer andthe powder material. By performing the compaction process in thismanner, the compact will not be in direct contact with the surface ofthe resin layer, and therefore, the adhesion between them may bereduced. As the cloth member, a filter cloth (such as felt), which isgenerally used in a wet molding process, may be used.

In the uniaxial pressing process described above, a magnetic field ofabout 1.3 MA/m is applied by the magnetic field generating coils 206perpendicularly to the uniaxial pressing direction (i.e., the press axisdirection).

In the compact obtained in this manner, chipping, cracking ordeformation has rarely occurred and the orientation directions of alloypowder particles have also been aligned sufficiently under the magneticfield.

The compact obtained in this manner is sintered at a temperature ofabout 1,000° C. to about 1,180° C. for approximately 1 to 2 hours, forexample. Then, the resultant sintered body is subjected to an agingtreatment at a temperature of about 450° C. to about 800° C. forapproximately 1 to 8 hours, thereby obtaining an R—Fe—B based sinteredmagnet. To reduce the amount of carbon included in the sintered magnetand thereby improve the magnetic properties thereof, the lubricant thatcovers the surface of the alloy powder is preferably burned off beforethe compact is subjected to the sintering process. The burn off processmay be carried out at a temperature of about 200° C. to about 600° C.and at a pressure of about 2 Pa for approximately 3 to 6 hours.

A compact having a uniform density distribution is obtained by aninventive method for producing a magnet. Thus, the compact hardly chips,cracks or deforms through the sintering process. As a result, a sinteredmagnet exhibiting excellent magnetic properties can be produced at ahigh productivity.

Hereinafter, the effects of the inventive powder pressing method will bedescribed with reference to FIGS. 10( a) and 10(b). FIG. 10( a) showsthe results of estimated variations in the size of sintered bodiesobtained by the method for producing a magnet according to theembodiment described above along with the results that were estimatedfor sintered bodies obtained by the conventional manufacturing process.FIG. 10( b) is a schematic representation showing a method forestimating a size variation.

In producing the sintered bodies representing examples of the presentinvention, the upper punch 205 shown in FIG. 4 was used as the upperpunch of the powder compacting apparatus 200. On the other hand, inproducing the sintered bodies representing conventional examples, anupper punch with a press surface made of a stainless steel (SUS 304) andincluding no resin layer 212 was used instead of the upper punch 205 ofthe powder compacting apparatus 200.

In FIG. 10( a), the abscissa represents the Shore A hardness of theresin layer 212 with the results of the punch with no resin layer (i.e.,conventional examples) shown on the right hand side. On the other hand,the ordinate of FIG. 10( a) represents the size variation Rav (mm).

As the materials of the resin layer 212, a silicone rubber with a ShoreA hardness of 25, urethane rubbers with Shore A hardnesses of 60, 70 and90, respectively, and a resin with a Shore A hardness exceeding 100(e.g., Jurakon™) were used.

The size variation R was obtained in the following manner.

First, as shown in FIG. 10( b), 15 measuring points are set for eachsintered body 30, and the difference between the maximum and minimumthicknesses (will be referred to herein as the variation R) that weremeasured in each of the magnetic field direction (at three points), thefeeder moving direction (at five points) and the thickness direction (at15 points) was obtained. These size variations R were obtained in therespective directions for each of the five sintered bodies 30 and theaverage thereof was regarded as the size variation Rav.

As is clear from FIG. 10( a), when a resin layer with a Shore A hardnessof 90 or less was used, the size variations Rav were smaller in themagnetic field direction and in the feeder direction than the situationwhere no resin layer was used or the situation where a resin layer witha Shore A hardness exceeding 100 was used. Conversely, the sizevariation Rav in the thickness direction increased in the situationwhere the resin layer with the Shore A hardness of 90 or less was used.These results show that the resin layer with the Shore A hardness of 90or less was deformed elastically in accordance with the non-uniform filldensity distribution during the uniaxial pressing process. Also, thesize variation Rav in the thickness direction in the situation where theresin layer with the Shore A hardness exceeding 100 (e.g., Jurakon™) wasused was almost the same as the situation where no resin layer wasprovided. Thus, it can be seen that the resin layer with the Shore Ahardness exceeding 100 was hardly deformed elastically during thepressing process and did not sufficiently counterbalance the non-uniformfill density distribution.

Furthermore, if a resin layer with a Shore A hardness of 70 or less wasused, the size variations Rav were substantially constant, small valuesin the magnetic field direction and in the feeder direction, while thesize variation Rav in the thickness direction increased as the Shore Ahardness decreased. That is to say, when a resin layer with a Shore Ahardness of 70 was used, the size variations Rav in the magnetic fielddirection and in the feeder direction were sufficiently small and thesize variation Rav in the thickness direction could be relatively small.Thus, it is believed that the preferred Shore A hardness range of theresin layer is preferably defined by the Shore A hardness of 70 as itscenter value, i.e., from 60 to 85.

FIG. 11( a) shows the outer (peripheral) shape of a sintered body, whichwas obtained by using a resin layer with a Shore A hardness of 70, asviewed in the press axis direction. FIG. 11( b) shows the outerperipheral shape of a sintered body, which was obtained by using anupper punch with no resin layer, as viewed in the same direction.

In each of FIGS. 11( a) and 11(b), the bold line represents thedeviation of the outer peripheral shape of its associated sintered bodyfrom the predetermined outer peripheral shape as indicated by the solidline. The deviation is herein exaggerated fivefold. The outer peripheralshape of each sintered body was obtained as the trace of an instrument60, which was moved in the direction indicated by the arrow in FIG. 12,for example, while keeping contact with the side surfaces of thesintered body 30 as shown in FIG. 12.

As is clearly seen when the results shown in FIGS. 11( a) and 11(b) arecompared to each other, the sintered body obtained by the inventivemanufacturing process is much less distorted than the sintered bodyobtained by the conventional manufacturing process. These results showthat a compact with a uniform density could be obtained by performingthe uniaxial pressing process using an appropriately elasticallydeformable resin layer.

As can be seen, in the sintered body obtained by the inventivemanufacturing process, just one surface thereof that has been in contactwith the elastically deformable press surface during the pressingprocess is uneven, while the other surfaces thereof have thepredetermined shape (i.e., flat). Accordingly, by grinding only thesurface that has been in contact with the elastically deformable presssurface, a sintered body with predetermined sizes and shape can beobtained. In contrast, the sintered body obtained by the conventionalmanufacturing process has all of its surfaces distorted significantly asshown in FIG. 11( b). For that reason, to obtain a sintered body withthe predetermined sizes and shape, all of those surfaces need to beprocessed. Thus, according to the manufacturing process of thisembodiment, just one surface needs to be processed and the throughputcan be increased. In addition, the processing margin (or the grindingmargin) can be reduced and the yield of the material also increases.

INDUSTRIAL APPLICABILITY

The present invention provides a powder compacting method that can makea compact with a uniform density distribution at a high productivityeven from a powder material with a non-uniform fill densitydistribution, and also provides a powder compacting apparatus that canbe used effectively to carry out such a powder compacting method. Inparticular, according to the powder compacting method of the presentinvention, a thin compact can be obtained at a high productivity from apowder material with a low flowability.

The powder compacting apparatus of the present invention can be easilyobtained just by making the press surface of a conventional uniaxialpress (i.e., a die press) of a resin layer with an appropriate hardness,for example. Thus, the present invention can be carried out easily.

Furthermore, the powder compacting method of the present invention makesit possible to produce a compact with a uniform density from a rareearth alloy powder that has been prepared by a strip casting process.Thus, the present invention provides a method for producing a rare earthsintered magnet at a high productivity.

1. A method for producing a magnet, comprising the steps of: providing apowder material including a rare earth alloy powder; loading the powdermaterial into a cavity; uniaxially pressing the powder material, whichhas been loaded into the cavity, between two opposed press surfaces,thereby obtaining a compact, wherein at least one of the two presssurfaces is deformed elastically under a compacting pressure whencontacting with the powder material in the cavity; and unloading thecompact from the cavity.
 2. The magnet producing method of claim 1,wherein the at least one press surface is the surface of a resin layer.3. The magnet producing method of claim 2, wherein the resin layer has aShore A hardness of 25 to
 90. 4. The magnet producing method of claim 1,wherein in the uniaxially pressing step, just one of the two presssurfaces is deformed elastically under the compacting pressure.
 5. Themagnet producing method of claim 1, wherein the loading step includesthe step of measuring the powder material with the cavity.
 6. The magnetproducing method of claim 1, wherein the loading step includes the stepof filling the cavity with the powder material at a relative density of0.20 to 0.35.
 7. The magnet producing method of claim 6, wherein theuniaxially pressing step includes the step of uniaxially pressing thepowder material to a volume that is 0.5 to 0.65 time as large as thecontent volume of the cavity.
 8. The magnet producing method of claim 1,wherein the compact satisfies D≦|S^(1/2) |/3, where D is the thickness(mm) of the compact as measured in a press axis direction in theuniaxially pressing step and S is the area (mm²) of each of the twopress surfaces.
 9. The magnet producing method of claim 1, furthercomprising the step of aligning the rare earth alloy powder by applyinga magnetic field thereto perpendicularly to the press axis directionduring the uniaxially pressing step.
 10. The magnet producing method ofclaim 1 wherein in the uniaxially pressing step, the press axisdirection is defined vertically, the two press surfaces consist of anupper press surface and a lower press surface, the side surface of thecavity is defined by an inner surface of a die, and the bottom of thecavity is defined by the lower press surface.
 11. The magnet producingmethod of claim 1, further comprising the steps of: sintering thecompact to obtain a sintered body; and finishing the surface of thesintered body, wherein the surface finishing step includes the step ofselectively grinding only a surface of the sintered body that contactedwith the at least one press surface in the uniaxially pressing step. 12.A powder compacting apparatus for uniaxially pressing a powder materialthat has been loaded into a cavity, the apparatus comprising: a diehaving an inner surface that defines the side surface of the cavity; alower punch having a lower press surface that defines the bottom of thecavity; and an upper punch having an upper press surface that is opposedto the lower press surface, wherein in uniaxially pressing the powdermaterial, which has been loaded into the cavity, between the lower andupper press surfaces, at least one of the lower and upper press surfacesis selectively deformed elastically under a compacting pressure amongthe inner surface, the lower press surface and the upper press surfacethat define the cavity, the at least one press surface is the surface ofa resin layer, the upper press surface is deformed elastically under thecompacting pressure, the upper punch includes a resin layer, portion ofwhich changes its hardness in the press axis direction, and the upperpress surface is the surface of the resin layer.
 13. The powdercompacting apparatus of claim 12, wherein the resin layer has a Shore Ahardness of 25 to
 90. 14. The powder compacting apparatus of claim 12,wherein just one of the lower and upper press surfaces is deformedelastically under the compacting pressure.
 15. The powder compactingapparatus of claim 12, wherein the upper press surface is to surface ofa resin layer, and wherein the upper punch includes a member forpreventing the resin layer from expanding in a horizontal direction,which is perpendicular to the press axis direction, under the compactingpressure.
 16. The powder compacting apparatus of claim 15, wherein theupper punch includes a concave portion to receive the resin layer, andwherein the side surface of the concave portion prevents to resin layerfrom expanding in the horizontal direction that is perpendicular to thepress axis direction under the compacting pressure.
 17. The powdercompacting apparatus of claim 12, wherein the resin layer includes: afirst resin layer with a first hardness; and a second resin layer with asecond hardness that is lower than the first hardness, and wherein theupper press surface is the surface of the first resin layer.